The present invention relates generally to elongated semi-finished, free machining steel articles and more particularly to such articles which are free of surface cracking and to methods for producing such articles.
A semi-finished steel shape is a bloom or a billet, the latter having a smaller cross-section than the former.
The machinability of steel is increased by adding alloying ingredients such as sulfur, lead, tellurium and bismuth.
In one type of manufacturing operation for producing a semi-finished steel shape having a free machining composition, molten steel, containing at least some of the machinability increasing ingredients described above, is poured into an ingot mold where the steel is cast into an elongated solid shape. After cooling, the ingot is reheated and subjected to a hot rolling operation which rolls the ingot into a bloom, and the bloom is then reheated and subjected to a further hot rolling operation to form the bloom into a billet. The billet is subsequently rolled into a bar, which is a finished steel shape.
In another form of manufacturing operation, the steel is continuously cast as strands having the cross-section of a billet, the strand is subjected to a roll bending operation or a roll straightening operation while the steel is at a hot deforming temperature, and, after the roll straightening operation, the strand is cut into billet lengths.
The hot rolling of the ingot into a bloom and the hot rolling of the bloom into a billet are generally performed at a hot rolling temperature in the range 920.degree.-1150.degree. C. (1700.degree.-2100.degree. F.). The roll straightening of a continuously cast strand is generally performed at a temperature in the range 920.degree.-1100.degree. C. (1700.degree.-2000.degree. F.). The hot rolling of the ingot into a bloom, the hot rolling of the bloom into a billet, and the roll straightening of the continuously cast strand are all hot deforming operations, and the temperature ranges described above are the typical, normal temperature ranges used for these hot deforming operations.
When tellurium is present in a free machining steel, all the tellurium is in inclusion form, principally as lead telluride (PbTe) when lead is also present. Some of the tellurium may also be present as manganese telluride (MnTe).
In steels containing lead and tellurium, an undesirable phenomenon occurs at the temperature range normally utilized for hot deforming. This phenomenon, known as surface cracking or surface checking or surface tearing, is manifest by a large number of small cracks or checks at the surface of the semi-finished steel article, particularly along the corners. Severe surface checking renders a steel article commercially unacceptable.
Surface cracking has been recognized in the prior art, and attempts to combat this phenomenon have also been described in the prior art. More specifically, attempts to combat surface cracking include reheating the unfinished steel article without flame impingement on the surface of the unfinished article (U.S. Pat. No. 3,287,954 Schrader et al), reducing the moisture content in the atmosphere of the reheating furnace (U.S. Pat. No. 3,365,922 Conces et al), removing the surface and first sub-layer of the unfinished steel article, e.g. by scarfing, after reheating (U.S. Pat. No. 3,382,700 Heitmann et al) and providing a blanket of nonoxidizing gas around the steel articles as they undergo reheating (U.S. Pat. No. 3,710,608 Hentz et al).
Typical examples of steel containing both lead and tellurium are disclosed in Holowaty U.S. Pat. No. 3,152,889.